Black liquor occurs as a by-product of alkaline fractionation processes of woody biomass, e.g. wood, straw, woody grass, etc. During a fractionation process, the lignin present in the woody biomass is brought into solution under alkaline conditions typically at temperatures up to 170° C. and then separated off from the components of the woody biomass that are insoluble under these conditions. An example of such an alkaline fractionation process is the production of chemical pulp by the KRAFT method in a chemical pulp factory. In addition to the dissolved lignin, black liquor contains further organic components and the predominant fraction of cooking chemicals used in the fractionation process. These are generally recovered in what is termed the chemical recovery of the fractionation process. The chemical recovery from the black liquor proceeds in the prior art substantially in two steps, wherein at first an evaporation and thus associated therewith a concentration of the dry matter content of the black liquor proceeds. Subsequently, the lignin fraction of the black liquor is combusted in a recovery boiler and the combustion residues are treated and the cooking chemicals present therein are thus recovered. The energy released in the combustion of the lignin fraction is used for heat and electricity generation.
In some chemical pulp factories, the recovery boilers operate at their maximum possible performance limit, in such a manner that a further capacity increase of the chemical pulp factory can only be achieved by installing a new recovery boiler or reducing the amount in the black liquor of the lignin that is to be combusted, e.g. by separating off the lignin from the black liquor.
Lignin can be separated off from black liquor before combustion thereof, for example by precipitating the lignin by means of gases having an acidic reaction with the black liquor, organic or inorganic acids followed by a solid-liquid separation. The crude lignin thus extracted, on account of impurities that are still present, in particular owing to the cooking chemicals, has an ash fraction of 15 percent by mass to 30 percent by mass (15 mass %-30 mass %) and must generally be disposed of as waste material. It has turned out that this crude lignin, by means of a further wash, for example with an acid, can be freed from inorganic impurities to the extent that it can, for example, be used as fuel in cement factories or energy generation plants.
Typically, however, lignin is not separated off from the total amount of black liquor which occurs in an alkaline fractionation process. Rather, of the total amount of black liquor that occurs, only as much black liquor is fed to a method for separating off lignin as is required to achieve a required relief of the recovery boiler. In an alternative approach, as a maximum, as much black liquor is fed to the method for separating off lignin such that, in the recovery boiler, from the remaining black liquor, still sufficient energy can be extracted for operation thereof.
In an optimized two-step method according to this teaching (LignoBoost method), the pH of the black liquor is first lowered slightly to about 9.5-10.5 by means of carbon dioxide and some of the lignin is precipitated in the course of this. Subsequently, lignin and black liquor are separated from one another in a mechanical dewatering. The black liquor that is only slightly reduced in pH is conducted, after the precipitated lignin has been separated off, back to the chemical pulp factory. The mechanically dewatered lignin is resuspended with water and the pH of the suspension is adjusted to about 2 using sulfuric acid. The suspension is then mechanically dewatered, the filter cake that forms is washed with acidic wash water and in this manner an ash content in the filter cake of below 5 mass % is reached. The filtrate from the second mechanical dewatering is generally recirculated to the chemical pulp factory in order to be able to recover the cooking chemicals present therein. The wash water is generally used to produce the suspension after the first dewatering. An advantage of such an optimized method procedure is ensuring an optimum recirculability of the black liquor after the precipitated lignin is separated off owing to the only slight acidification in the first method step and also the low ash content of the lignin after the acid wash in the second method step, which permits the lignin to be marketed, e.g. as fuel. The consequences of integrating such a method on the balance of the cooking chemicals of the chemical pulp factory are a disadvantage, in particular when the filtrate from the second dewatering step and therefore the sulfur present therein, are recirculated to the chemical pulp factory. Also, the high operating costs that are caused by using carbon dioxide and the necessity for separating off the sulfur introduced with the sulfuric acid, and also the low revenues which can be earned when using lignin as fuel, are disadvantageous. This prior art is given, for example, in WO 2013/070130 A1, WO 2013/002687 A1, WO 2012/177198 A1, WO 2010/143997 A1 or WO 2009/104995 A1.
For further treatment of the lignin that is separated off from the black liquor and purified by the abovedescribed optimized two-step method to give a value-added solid carbon, it is suitable to feed the purified lignin to a hydrothermal carbonization and thus refine it to give a solid carbon.
According to the prior art, organic material is treated in a hydrothermal carbonization at temperatures between 150° C. and 300° C. in the presence of liquid water and at a pressure which is above the saturated vapor pressure for a time from 30 minutes to 24 hours. The reaction water, before the start of the hydrothermal carbonization, generally has a neutral pH or an acid pH by addition of an acid. After completion of a hydrothermal carbonization, the pH is markedly in the acid range. According to the prior art, a hydrothermal carbonization is catalyzed by the addition of acids, e.g. citric acid. The acids forming from the biomass during a hydrothermal carbonization also act autocatalytically. The result of a hydrothermal carbonization is a solid carbon which has an increased carbon content and a reduced oxygen content in comparison with the starting material (WO 2010/112230 A1).
In experiments it has now been found that lignin that is extracted from black liquor by the abovedescribed two-step method (LignoBoost method), which is subjected as feedstock to a hydrothermal carbonization, forms during this solid deposits on the reaction vessel used that hinder a production operation or make it impossible. A further disadvantage of a hydrothermal carbonization of the lignin extracted from black liquor by the prior art is the expenditure on plants which results from the combination of the two-step precipitation and purification method with a hydrothermal carbonization of the prior art.
Direct hydrothermal carbonization of black liquor is not prior art. In the closest prior art (WO 2012/091906 A1), it is proposed to treat black liquor hydrothermally at a temperature between 250° C. and 300° C. and thus to reduce the water-insoluble fraction of solids by at least 40%. The aim of this prior art is to depolymerize the lignin in the black liquor by a hydrothermal treatment in order to be able to separate off the resultant water-soluble phenolic oligomers and monomers readily from solids by filtration and to be able to feed them as feedstock to subsequent chemical processes. This prior art, in the result, does not separate off the lignin from the black liquor.